In packaging technology heat-sealing is a conventional method for manufacturing or closing containers or packages made of polymer film or polymer-coated packaging material, such as paper, paperboard or cardboard. Low density polyethylene (LDPE) is a material commonly used in packages due to its easy heat-sealability. Furthermore, many other polymers are used in packages, for example polyesters, which, unlike LDPE, are biodegradable or have better water vapour and/or oxygen barrier properties than LDPE. These other polymers, however, are usually more difficult to heat-seal than LDPE, which is why they are not readily positioned as a surface layer but rather as an inner layer of a multilayer packaging material.
Polyethylene terephthalate (PET) is a polyester, which is frequently used in packages and containers, has good barrier properties, and is very heat-resistant, which is why it is suitable for the coating of ovenable food containers or packages or of baking cardboard, for instance. A disadvantage is that PET is difficult to heat-seal. Moreover, conventional PET is non-biodegradable.
A biodegradable polymer that is commonly used in biodegradable packages consisting of polymer film or polymer-coated paper or board is polylactide (PLA). Polylactide has relatively good water vapour and gas barrier properties, but has the problems of weak adhesion to a fibrous substrate and a high melting temperature, resulting in poor heat-sealability.
To improve the heat-sealability of polylactide US 2002-0065345 A1 describes blending of polylactide with a biodegradable aliphatic polyester made from a diol and a dicarboxylic acid, for example polycaprolactone (PLC) or polybutylene succinate adipate (PBSA), the portion of which in the mixture is at least 9%.
According to US 2005-0192410 A1 processability of polylactide is improved by blending polycaprolactone and mineral particles into it. US 2007-0259195 A1 further describes polylactide-based films and polymer coatings, which are extruded onto a fibrous substrate and wherein polybutylene adipate terephthalate (PBAT) is blended with polylactide to improve its heat-resistance.
WO 2011/110750 describes a polylactide-based double-layer coating, which is extruded onto a fibrous substrate and in which the outer layer has a larger portion of biodegradable polyester (other than polylactide) blended therewith than the inner layer, with an aim to optimize the adhesion between polylactide and the fibrous substrate and the heat sealability of polylactide.
When the heat-sealability of polylactide is improved by another polyester or similar additive blended therewith, there is the disadvantage that these additives are more expensive than polylactide. Furthermore, the blending of polymers constitutes an extra working step in the complex process.
A different approach is represented by WO 2011/135182, which teaches ultraviolet (UV) radiation of a polyester layer to improve its heat-sealability. According to tests the heat-sealing temperature is decreased, but any explanation why this is happening is not given. It also seems that the favourable effect is rather limited to polylactide only.
WO 98/04461 teaches use of electron beam (EB) radiation for improving heat-seals of polyolefins, such as low density polyethylene (LDPE), on a paperboard substrate. EB is said to induce cross-linking of the polymer and thus increase the molecular weight of same. The melt index of the polyolefin is significantly decreased, bringing an increase to melt viscosity and the melting point. Such an increase in fact detracts from the ease of heat-sealing by raising the heat-sealing temperature required, even though the strength of the seal may improve, which is the goal of these prior art teachings.
From the publications CN 101824211 A, CN 101735409 A and CN 101225221 A it is known to improve the heat-resistance of polylactide by subjecting it to electron beam (EB) radiation, which produces cross-linking while preserving biodegradability of the material. Cross-linking is secured by addition of a catalyst such as triallyl isocyanureate (TAIC). The prior art teachings relate to moulded articles or granules, but not to coatings on a fibrous substrate, in which adhesivity to the substrate and heat-sealability are required. As cross-linking increases the molecular weight and the melt viscosity of the polymer it would be expected to have an adverse effect on heat-sealability.